1. Field of the Invention
The present invention relates to the field of laser beam delivery systems, particularly to laser beam endoscopic focusing guide for use in laser surgery.
2. Description of Prior Art
At the present time laser techniques find increasing medical applications, in particular in laser surgery. To operate with a laser, a laser beam should be delivered to the operation site and converted into other forms of energy, such as heat or acoustic energy which is concentrated within a specific volume. However, operation sites usually are located remotely from the source of laser energy and are very often poorly accessible.
Optical systems used for delivering laser energy to the operation or treatment site are known as laser beam delivery systems. In case the laser operates in the visible wavelength range, it can be delivered through an optical fiber link. For surgical applications the optical fiber link is inserted, together with other surgical supplements, into a protective guiding tube which is known as an endoscopic tube.
An optical fiber or an optical fiber link normally consists of a central core portion made of a transparent, low-energy-loss material with a high refractive index, an intermediate layer known as cladding, which is made of a material having a refractive index lower than that of the core, and an outer or protective layer which is known as buffer, and which can be made of various materials, e.g., of a plastic. Light is propagated through such a fiber optical link due to well-known multiple total internal reflections. There is no leakage of light energy through the walls of the core to the outside because the core is made of a material having a higher refractive index than the outer layer.
In order to ensure propagation of the light through the optical fiber, it must be injected into the optical fiber under an incident angle, known as an acceptance angle. The acceptance angle should not exceed a predetermined threshold value, known as a critical angle. If this value is exceeded, the light will leak through the walls of the core to the outside.
Once the light is injected into the inlet end of the optical fiber at a predetermined critical angle, it will emerge, after multiple reflection from the core walls, from the outlet end face of the optical fiber at the same angle in the form of a diverging beam.
It is obvious that the energy density of the emerging light will have a maximum value at the very end of the optical fiber. Beyond the end face, the light's energy density will quickly dissipate.
For better understanding the objects and principle of the present invention, several examples of fiber optic laser techniques in surgery will be considered with reference to the principle of distribution of laser energy over the operation site.
Let us consider the case when an optic fiber is used for an operation such as thermal treatment, e.g., in dermatology, or for ablation, e.g., in surgery. Ablation is a removal of a part, such as a tumor. In order to remove a tumor by a laser beam, the tumor must be heated to a vaporization point for removing its substance by evaporation. However, when the tumor is heated, a portion of heat and light energy penetrates the surrounding healthy tissue and creates necrosis, i.e., tissue death, in this zone.
The energy supplied to the operation site can be adjusted by changing the distance between the front end of the fiber core and the surface to be treated. When this distance is increased, the energy density is reduced, and vice-versa. However, in the case described above, i.e., for the removal of the tumor, this distance cannot be considerably reduced, as the temperature will exceed the vaporization point of the tumor substance. This will result in burning and carbonization of the tumor tissue, instead of evaporation. Thus, the adjustment of energy by changing the distance is limited.
The situation is aggravated when a tumor is located inside an organ. This is because, in the course of penetration of the laser beam toward the tumor, the beam looses a considerable part of its energy and also destroys a considerable amount of the surrounding healthy tissue.
Irrespective of whether the treated tumor is on the surface or inside the organ, the process is always accompanied by a phenomenon known as "back scattering", i.e., returning of a portion of heat energy back to the fiber core. This back scattering accelerates destruction of the core.
In some cases an optical fiber can be used as a tool for cutting tissue by placing it in direct contact with the tissue. When the fiber core is used for cutting a tissue, the fiber core is moved across the operation site in the cutting direction. Such cutting is known as a "dragging" operation. Dragging is performed, not by the entire end face of the fiber core, but rather by a hot point at its leading edge. Thus the laser beam energy is utilized with extremely low efficiency, because its major part is dissipated directly into the surrounding tissue and is not used for cutting.
In other cases, laser surgery is carried out by means of an optical fiber with a sapphire tip attached to its end.
An advantage of such an instrument in comparison to the one without a sapphire tip, i.e., with a flat end, is that the sapphire tip will concentrate almost the entire energy at the point of contact with the tissue. However, an instrument of this type is efficient only for cutting and is unsuitable for operations requiring either a lower operation temperature or a larger treatment area.
Optical fibers are also used in laser angioplasty, i.e., the plastic surgery of diseased blood vessels. In laser angioplasty an optical fiber is inserted into a blood vessel, moved along the vessel, and used, e.g., for removing plaque from the inner walls of the vessel.
At the present time, however, optical fiber laser angioplasty can be carried out only in relatively short and straight vessels because existing techniques suitable for such operations allows the beam to exit only in a straight-forward direction.
In addition, when plaque areas are located asymmetrically, i.e., not opposite to each other on the inner wall of the vessel, simultaneously with the removal of a plaque area, the straight-forward beam will damage the opposite inner wall.
Thus, in laser angioplasty the laser beam's energy is used inefficiently because the laser instrument is aimed at treating lateral objects while the energy beam is aimed in a forward direction. Also, the optical fiber is located in a narrow blood vessel and therefore cannot be bent. This is because, in a bent state, an optical fiber may preserve its operation characteristics only when the radius of the curvature exceeds 3-4 cm, and this is impossible because of the limited space inside the blood vessel.
In a human body, however, none of the blood vessels is ideally straight and some of them have very intricate paths, forming V-or U-shaped configurations. In order to treat hard-to-reach areas in such vessels, the laser beam should be guidable throughout a wide range of angles. The same is true, not only for angioplasty, but also for other types of laser surgery.
A more detailed description of optical fiber laser surgery techniques is given in "Optical Fibers in Medicine" (SPIE [Society of Photographic Instrumentation Engineers] 1990), Volume MS 11, Bellingham, Wash., USA.